The present invention relates to an improvement for a circuit for muting a pulsy noise which is generated when a power supply of a power amplifier is turned on.
In an audio system, at the time when a power source of a power amplifier is turned on and off, a large pulsy noise such as a pop emitted from speakers. The noise is caused by an imbalance of operational timings of the power supply voltages of direct current (+Vcc, -Vcc), and when a capacitor of a feedback loop is charged. Such a transitional noise not only impairs the listening atmosphere but may further damage expensive loudspeakers.
In order to prevent the pulsy noise, there has been proposed a protecting circuit having a relay circuit together with a delay circuit between the power amplifier and the loudspeaker.
Referring to FIG. 4 showing a conventional protecting circuit, when a power switch 101 is closed, an alternating voltage is transmitted through a transformer 102 to a power supply circuit 103 comprising a rectifier and a smoothing circuit. The power supply circuit 103 applies a predetermined voltage to a power amplifier 105 to which an audio signal is fed through an input terminal 106. The audio signal is amplified and applied to a loudspeaker (not shown) through a relay circuit 108 and an output terminal 109. The relay circuit 108 has a relay and a delay timer.
The protecting circuit further has a power-on detecting circuit 104 comprising a rectifier and a comparator. The detecting circuit 104 applies a power-on signal to the delay timer of the relay circuit 108.
When the power switch 101 is turned on, the direct voltage is applied to the power amplifier 105, thereby operating it. At the same time, the alternating voltage is applied to the power-on detector 104 which accordingly applies the power-on signal to the delay timer of the relay circuit 108. After a few second from the time when the power-on signal is fed, the relay switch is closed, thereby applying the audio signal from the power amplifier 105 to the terminal 109.
Transistors and capacitors provided in the power amplifier 105 are actuated during these few seconds while the relay is kept opened. Hence the occurrence of pulsy noise is prevented.
However, the relay is expensive and moreover, unreliable. In addition, the use of the relay renders it difficult to reduce the size of the electronic devices such as the power amplifier. Furthermore, the relay decreases the quality of the sound emitted from the speaker.